The biological functions of WRKY transcription factors in plants have been widely studied, but their roles in abiotic stress are still not well understood. We isolated an ABA overly sensitive mutant, abo3, which is disrupted by a T-DNA insertion in At1g66600 encoding a WRKY transcription factor AtWRKY63. The mutant was hypersensitive to ABA in both seedling establishment and seedling growth. However, stomatal closure was less sensitive to ABA, and the abo3 mutant was less drought tolerant than the wild type. Northern blot analysis indicated that the expression of the ABA-responsive transcription factor ABF2/AREB1 was markedly lower in the abo3 mutant than in the wild type. The abo3 mutation also reduced the expression of stress-inducible genes RD29A and COR47, especially early during ABA treatment. ABO3 is able to bind the W-box in the promoter of ABF2in vitro. These results uncover an important role for a WRKY transcription factor in plant responses to ABA and drought stress.